1. Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a fuel supply unit of a fuel cell system.
2. Description of the Related Art
In general, a fuel cell is an electricity generating system directly converting chemical energy into electrical energy through an chemical reaction between oxygen contained in the air and hydrogen contained in hydrocarbon-containing materials such as methanol and natural gas.
Commonly used fuel cells include a phosphate fuel cell, a molten carbonate fuel cell, and the like. Recently, a polymer electrolyte membrane fuel cell (PEMFC) has been developed as a highly efficient fuel cell.
The polymer electrolyte membrane fuel cell has an excellent output characteristic, a low operating temperature, and a fast starting and response characteristic compared to other fuel cells. It uses hydrogen obtained by reforming methanol, natural gas, etc. Accordingly, the PEMFC has a wide range of applications such as a mobile power source for vehicles, a distributed power source for the home or buildings, and a small-sized power source for electronic apparatuses.
The aforementioned PEMFC basically comprises a fuel cell main body called a stack, a fuel tank, and a fuel pump supplying fuel to the stack from the fuel tank. When methanol is used as the fuel, such a fuel cell further comprises a reformer which converts the methanol to generate hydrogen.
The stack comprises a membrane-electrode assembly (MEA) having an anode, a cathode, and an electrolyte membrane integrally formed with the electrodes, and a separator (or a bipolar plate) for electrically connecting the MEAs to each other at the time of stacking a plurality of MEAs.
Alternatively, the a fuel cell can employ a direct methanol fuel cell (DMFC) scheme which directly supplies liquid-state fuel containing hydrogen to the stack and to generating electricity. The fuel cell employing the DMFC scheme does not require the reformer, unlike the PEMFC.
As the fuel cell system operates, water is generated as a byproduct in a chemical reaction in the stack causing a gas passage to be blocked or a surface thereof to be corroded by the water, which leads to a decrease in efficiency. Hence, the separator needs to have strong corrosion resistance. To prevent the separator from being deteriorated in the conventional fuel cell system, a method of vaporizing the water generated in the stack has been studied. However, research has demonstrated that a solid oxide fuel cell (SOFC) working at a high temperature of about 600 to 1,000° C. could be developed, but the same result could not be achieved for the PEMFC or the DMFC working at a relatively low temperature of about 60 to 100° C. or from a room temperature to 45° C. Prior to this invention, it has not been possible to reliably prevent the separators, from being damaged due to water generated in the stack in the proposed fuel cell system.